I'm looking to change the valves on my Origin 50, 2 years old, loads of use. May have been dealt with before but can anyone advise on biasing the output valves please. I was planning to get a matched pair of EH's, will they need biasing? I have a mate who can do it if so, but he's struggling to find a schematic of the 50. Love the amp, run into a Blackstar Cab with Vintage 30's in. Thanks in advance.